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February 25, 2002, 15:43 
Generate solution (fluent)

#1 
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I have been struggle to generate result for my model. However, when i try to iterate my model first with ke model then using laminar model, it suprrisingly solved... my question  are these steps were allowed? will it generate a fault result? i know this question sound silly, but i really need your help. i am new in using the Fluent thank you


February 26, 2002, 00:39 
Re: Generate solution (fluent)

#2 
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In general, for turbulnt flow, it is impossible to obtain cponverged solution by laminar solver. However, if you obtain two different solution by laminar solver and by turbulent solver, two results should be different from each other. Especially, for laminar flow, if you use turbulent option, you can in general easily obtain the converged solution. But it is very unreallistic result.
Actually speaking, as a concept, all turbulent flow can be solved by timedependent laminar solver. This is the concept of the DNS(Direct Numerical Simulation). And there might be two different solutions in the real situation, one belong to turbulent regime and one belong to laminar regime. However, this is fairly difficult topic for the beginner. It is related to the hydrodynamic stability, the degree of pertubation and so on. So, just refer this comment in the future. And at this time, for the beginner, I can say that two flow regime are separated, so that, first of all, you should check that your flow is turbulent or lamninar. And use appropriate option(laminar or turbulent). Sincerely, Jinwook 

February 26, 2002, 06:54 
Help agian.. Generate solution (fluent)

#3 
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Thank you to Jinwook Lee.
My situation is like this. I am working on Jet impingement, and require to determine the pressure distribution along a flat surface which is normal to the air jet. I am using Gambit and Fluent. My model based on low Re. no. less than 2000. I have no problem to get the solution solved (using laminar model) when the distance between the jet exit to a nonmoving flat surface is r(radius) and 5r. However, when i increase the distance between the jet exit and the nonmoving flat surface to 10r and 25r, it can't be converge(using laminar model). Jet velocity inlet = 5m/s Air density = 1.225kg/m2 To all the Gurus, senior....i need hint and guide line when dealing with this problem. @@I feel sad as i can't get it right. However, now i am more 'addicted' to CFD, is this kind of strange?? @@ 

February 26, 2002, 07:36 
Re: Help agian.. Generate solution (fluent)

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Impinging jets tend to become unsteady very easily, you might even have transition occuring when you increase the jetlength. This would of course make it difficult to converge  try running it unsteady. You should also compute the Reynolds number of the jet to see if you can expect transition  wihtout having done any calculations on it a jetlength of 25r sounds long to be laminar, are you sure it should be laminar? On which dimensions did you base the Re=2000 figure?


February 27, 2002, 06:27 
Re: Help agian.. Generate solution (fluent)

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Thank You Jonas Larsson. After reading your reply, i had make a double check on my calculation on the re. no. For my model(1 of the unsolved), distance between jet and flat nonmoving surface = 10r distance spacing between jets = 10r Air density = 1.225 kg/m^2 velocity = 5m/s viscosity = 1.7894 x 10^5 Re.no = (density x velocity x length)/viscousity Assumption r=2mm
I had tried using the unsteady model, still it can't solved.. 

February 27, 2002, 09:59 
Re: Help agian.. Generate solution (fluent)

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My guess is that you are passed the critical Reynolds number  for pairs of jets with no impingment you will start getting Hopfbifurcations and unsteady/chaotic flow at Reynolds numbers around something like 50 (based on jet diameter) I think. In your case your Reynolds number based on jet diameter is 400 if I interpret your formulas right (I assumed that "length" in your Re number formula is 10r).
Hence, you are most likely entering an unsteady transitional flow regime which is very difficult to simulate using a laminar steady approach. 

February 27, 2002, 12:00 
Re: Help agian.. Generate solution (fluent)

#7 
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thank you, Jonas Larsson. I will try to solved it by reducing the velocity inlet, and make sure the flow will be in the laminar region. i hope to obtain solution soon...


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